Note that neither the "see full text" link nor the scrollbar for the second message is present in the bookmarked conversation, although for the first the link appears.

I'm not sure what the key difference between the two messages is.

Workaround is to click the permalink for the message missing the full text link to go to the transcript, where it can be viewed, assuming you already know that it's clipped: Since the "see full text" link and scrollbar are both missing from the second message, there isn't really any indication that there's more text there to begin with.

Incidentally, the presence of the scrollbar seems inconsistent, or at least is not predictable to me. I'm not sure why the first message has it but the second does not; the second has considerably more lines than the first.

"See full text" link never show in bookmarked conversation. (The scrollbar you found in conversation is actually because message has scrollbar which is different from "See full text" link). In other words, the behaviour of scroll-bar and "See full text" are different and 1st is shown in conversation whereas 2nd is not.
– PandyaOct 3 '16 at 6:21